There is a growing demand for small and light resonators and filters having the resonators, with the rapid spread of mobile communication devices such as a mobile phone unit. Recently, there is developed a filter for high frequency communication that has characteristics of allowing of passing only an electrical signal in a specific frequency range, the characteristics being obtained by combining a surface acoustic wave resonator having a piezoelectric material and a piezoelectric thin-film resonator using thickness vibration of a piezoelectric thin film. An outer shape of a filter using the surface acoustic wave resonator or the piezoelectric thin-film resonator is smaller than that of a dielectric filter. And a Q value of the surface acoustic wave resonator is higher than that of the dielectric filter. Therefore, the surface acoustic wave resonator is appropriate for a high frequency component of a mobile communication device such as a small mobile phone unit for which sharp roll-off characteristics are required. A duplexer is used as an applied component having a ladder type filter of the surface acoustic wave resonator or the piezoelectric thin-film resonator. The duplexer has a function of transmitting and receiving, and is used as a wireless device in which a transmitting signal frequency is different from a receiving signal frequency.
There are a FBAR (Film Bulk Acoustic Resonator) type and a SMR (Solidly Mounted Resonator) type as the piezoelectric thin-film resonator. The piezoelectric thin-film resonator of FBAR type has a lamination structure in which a lower electrode, a piezoelectric film and an upper electrode are laminated on a substrate in this order. A region sandwiching the piezoelectric film between the lower electrode and the upper electrode is a resonance portion. A cavity is formed below the lower electrode of the resonance portion. On the other hand, the piezoelectric thin-film resonator of SMR type has a structure in which an acoustic multilayer film is provided instead of the cavity. The acoustic multilayer film has a structure in which a film having higher acoustic impedance and having a thickness of λ/4 and a film having lower acoustic impedance and having a thickness of λ/4 are laminated (λ: a wavelength of an acoustic wave).
A filter or a duplexer using a piezoelectric thin-film resonator has temperature characteristics of frequency. As the performance of a mobile phone unit or the like gets higher, improvement of the temperature characteristics is needed, and reduction of an absolute value of a temperature coefficient of a frequency such as a resonance frequency or anti-resonance frequency is needed. Japanese Patent Application Publication No. 58-137317 (herein after referred to as Document 1) discloses that a thin film of which sign of a temperature coefficient is opposite to that of a piezoelectric film is provided between piezoelectric films as illustrated in FIG. 5, and an absolute value of the temperature coefficient of frequency is reduced. Qiang Zou, and six people, “High Coupling Coefficient Temperature Compensated FBAR Resonator for Oscillator Application with Wide Pulling Range”, Frequency Control Symposium, 2010 IEEE International, p 646-651 discloses that a silicon oxide film acting as a temperature compensation film is provided between a lower electrode and a piezoelectric film as illustrated in FIG. 2, a metal film covers the silicon oxide film, the temperature characteristics of frequency are improved, and an electromechanical coupling coefficient (K2) is improved.
In a piezoelectric thin-film resonator, orientation of a piezoelectric film is one of important elements for determining resonance characteristics. However, the orientation of the piezoelectric film may be disturbed, as in the case of the structure of Document 1 in which the thin film acting as the temperature compensation film is sandwiched by piezoelectric films.